Today, we find a broad field of so-called chip arrays with covalently attached capture substance molecules for detecting affinity binding biopolymer molecules. The capture substance molecules bound to the array fields can be DNA molecules (“DNA chips”), protein molecules (“protein chips”) or other types of molecule which can exhibit affinity binding. In the following, the biopolymer molecules will be termed “analyte molecules”, the capture substance molecules on the chip fields simply as “probe molecules”. The hybridisation or affinity binding of the analyte molecules to the probe molecules takes place using solutions in which target analyte molecules can occur, the solutions being in direct contact with the coated chip surface.
These chip arrays with probe molecules are used to study the binding characteristics (for example the study of possible cross reactions in the case of antibody binding), but especially they serve for the selective capture of analyte molecules from body fluids and hence for the qualitative and quantitative analysis of these analyte molecules. In some cases, for example for the detection of characterizing DNA strands of infection pathogens, the analyses are limited to simple statements as to the presence or absence of the infectious pathogens. The multiplicity of probe fields on the chip arrays means that in a sample with body fluids, the presence of one or more from many different types of infectious pathogens can be simultaneously detected.
The analytical methods with such chip arrays are also termed cell-based assays; the method itself is often termed “screening”.
The chip arrays can be manufactured from semiconductor material such as silicon wafers, but any other type of wafer shaped materials can form the base to be coated with probe molecules. Different types of glass, metal or even plastics can be used for this. Semiconductors possess the advantage that electrical circuits can be incorporated into them, using microfabrication methods.
Only a small number of methods have so far been introduced as prior art for the detection of the binding of analyte molecules to probe molecules and they are only described very briefly here.
Detection of the binding is, for example, possible by using fluorescent dyes additionally bound to the analyte molecules; such methods require laser scanners or fluorescence microscopes. These instruments and methods are expensive mainly because the necessary (patented) fluorescent dyes are responsible for high consumable costs.
Mass spectrometric detection of the affinity bound analyte molecules, for example with ionization by matrix-assisted laser desorption (MALDI) after the addition of the appropriate matrix substances, is expensive because it requires a mass spectrometer. Consumable costs are restricted to the chip costs. Mass spectrometric detection does, however, have the advantage that it also provides additional confirmation of the identity of the analyte molecules by virtue of their mass.
A further method, currently under development, consists of the simultaneous binding of the analyte molecules and larger masses, for example using nanoparticles, onto suitable oscillators to detect the affinity binding by means of surface acoustic waves whose frequency depends on the mass of the coating.
The method of plasmon resonance spectrometry, which is also used to detect the affinity binding of analyte molecules to probe molecules, requires somewhat larger areas for the flat reflection of the light, so that it has not yet proved possible to produce arrays with large numbers of fields for this type of detection. The advantages lie in the fact that this method can also measure the kinetics of the binding process.
The use of chip arrays which can simultaneously detect the presence or absence of hundreds or thousands of different types of analyte substances by coating them with different kinds of probe molecules is predicted to have a great future. The existing readout methods are still too complicated, however. There is therefore a need for a simple method of reading out the binding of the analyte molecules as directly as possible. A simple readout method for affinity binding would not only be of interest for chip arrays with many fields, but also for individual fields which can be charged in succession with different types of ligand, for example.